Power systems such as electrical power distribution or transmission systems generally include a protection system for protecting, monitoring and controlling the operation and/or functionality of other components included in the power system. Such protection systems may for example be able to detect short circuits, overcurrents and overvoltages in power lines, transformers and/or other parts or components of the power system. The protection systems can include protection equipment such as circuit breakers for isolating any possible faults for example occurring in power transmission and distribution lines by opening or tripping the circuit breakers. After the fault has been cleared, e.g. by performing repairs and/or maintenance on the component in which the fault has been detected, the power flow can be restored by closing the circuit breakers. In alternative or in addition, the protection systems can be arranged to, upon detection of a fault in a particular route for power flow, isolate the route in which the fault has been detected and select an alternative route for the power flow.
Operation of the circuit breakers may be responsive to detection of a fault condition or fault current. Upon detection of a fault condition or fault current, a mechanism may operate the circuit breaker so as to interrupt the current flowing there through. Once a fault has been detected, contacts within the circuit breaker may separate in order to interrupt the current there through. Spring arrangements, pneumatic arrangements or some other means utilizing mechanically stored energy may be employed to separate the contacts. Hence, mechanical current interrupters may for example be employed in circuit breakers. In alternative or in addition, solid-state interrupters based on semiconductor devices may be employed in the circuit breakers. When interrupting the current flowing in the electrical circuit, an arc is in general generated. Such an arc may be referred to as a fault current arc. In order to break the current in the electrical circuit, it may be required or desired to extinguish such an arc. Once the fault condition has been mitigated or eliminated the contacts can be closed so as to resume flow of current through the circuit breaker.
High Voltage Direct Current (HVDC) power transmission is becoming increasingly important due to the increasing need for power supply or delivery and interconnected power transmission and distribution systems. An HVDC grid or a DC grid may comprise multiple alternating current (AC)/DC converter terminals interconnected by transmission lines, e.g., underground cables and/or overhead lines. Within the grid, a terminal may be connected to multiple terminals resulting in different types of topologies. DC circuit breakers can be used for isolating faulty components, such as transmission lines, in power systems such as HVDC and DC grids. Unlike AC circuit breakers, there are no natural current zeros at which a fault current arc may be extinguished in DC circuit breakers. Instead, it may be desired or even required to create one or more current zeros when utilizing DC circuit breakers.
One example of a way to create current zeros in order to extinguish a fault current arc in a current interrupter in a DC circuit breaker is to employ a so called resonance circuit connected in parallel with the current interrupter. The resonance circuit may in alternative be referred to as an oscillation circuit or an injection circuit. The resonance circuit can be operated such that an oscillation starts, wherein a high frequency current, or resonance current or injection current, created in the resonance circuit superposes the fault current and generates one or more current zeros, at which the fault current arc can be extinguished. The resonance circuit may for example comprise an inductor, a capacitor and possibly a switch element connected in series. During normal operation, i.e. when no fault has been detected or no fault current has been sensed, the current interrupter is closed and the switch element in the resonance circuit is open. Upon reception of a trip signal, which e.g. may be issued by an external control unit or a protective unit of an HVDC power transmission system in which DC circuit breaker is included, the current interrupter is opened by separating contacts therein to interrupt the direct current through the DC circuit breaker. Upon interrupting or breaking the direct current, a current is carried between the contacts of the current interrupter through an arc. Thus, a fault current arc is created between the contacts in the current interrupter. It is generally desired or even required to extinguish the fault current arc in order to break the current through the DC circuit breaker. A short time after the current interrupter has been opened, typically after about one or a few milliseconds, depending e.g. on how much the contacts have been separated, the switch element in the resonance circuit is closed. In order to start the oscillation after the current interrupter has been opened (and the switch element has been closed), the capacitor in the resonance circuit normally needs to be charged. The capacitor, being charged with a certain polarity, discharges via one of its ends or capacitor plates through the inductor in the resonance circuit. As the capacitor discharges, the inductor will create a magnetic field. Then, once the capacitor has been discharged, the inductor will charge the capacitor. Once the magnetic field of the inductor collapses, the capacitor has been recharged (but with the opposite polarity), and so it may discharge again through the inductor. The thus created resonance current can be made to superpose the fault current and generate one or more current zeros, at which the fault current arc can be extinguished. In alternative or in addition to charging the capacitor with a voltage to start the oscillation, a magnetic field may be created in the inductor by means of a current or a magnetic pulse. Hence, by creating a magnetic field in the inductor, the resonance circuit may be activated in order to start an oscillation that creates a resonance current.
However, there is still a need in the art for improved DC circuit breaking arrangements which can provide an improved performance with respect to operation compared to known DC circuit breaking arrangements.
EP 0 108 279 A1 discloses a high-voltage DC circuit breaker apparatus with reverse current insertion system by line voltage charge, which comprises a commutation switch, a series circuit of a capacitor, a reactor and a switch connected in parallel with the commutation switch, and a resistor (R) connected so us to charge the capacitor by a line voltage.
DE 20 38 624 A1 discloses a method and apparatus for breaking direct and alternating currents at high voltages. In parallel with a commutation switch is a series circuit of a capacitor, a reactor and a switch, as well as a resistor.